One of the largest California bushfires in decades has roared largely unchecked for a 10th day, moving perilously closer to a reservoir that provides most of San Francisco's water supply.

The eastern flank of the so-called Rim Fire blazed through forests in and around Yosemite National Park and burned to within 1.6 km of Yosemite's Hetch Hetchy Reservoir on the Tuolumne River.

That raised concerns about the possibility of ash contaminating the sprawling artificial lake.

Just two days earlier, flames had been no closer than about 6.5 km from the reservoir.

The reservoir supplies 85 per cent of the water consumed by 2.6 million people in San Francisco and several surrounding communities.

Reservoir samples show water quality so far remains healthy, according to Suzanne Gautier, a spokeswoman for the San Francisco Public Utilities Commission.

But the fire has already damaged two of the three hydropower generating stations linked to the reservoir that supplies electricity for all of San Francisco's public facilities, such as hospitals and firehouses.

Thousands of hectares scorched

Ms Gautier says for now the city continues to draw on reserve power stored for emergencies and has purchased additional electricity on the open market to make up for the difference.

Despite threats to water and power, firefighters have made headway against the blaze, although rugged terrain and the fire's remote location are complicating efforts to subdue the flames, fire managers said.

"There are places where they can't get in," said Mike Ferris, a spokesman for the National Interagency Fire Center in Boise, Idaho. "It's just inaccessible."

Fire crews working with hand tools and backed by bulldozers and water-dropping helicopters had carved containment lines around 15 per cent of the blaze's perimeter by Monday (local time).

That was more than double Sunday's figure, though the fire's footprint continues to grow.

The Rim Fire has charred about 377 square kilometres or 60,703 hectares - the size of the city of Chicago - since it erupted on August 17, according to the latest estimates.

It is the largest bushfire in California since August 2009, when the so-called Station Fire ravaged the Angeles National Park north of Los Angeles.

It is one of the 20 biggest in state history, according to the California Department of Forestry and Fire Protection.

Rim Fire cause unknown

The largest California blaze on record is the 2003 Cedar Fire, which scorched more than 110,000 hectares in San Diego County and destroyed more than 2,800 structures.

The Rim Fire has forced the closure of the main road leading into the Yosemite park from the San Francisco Bay area and has prompted the evacuation of 74 campsites in the park's White Wolf area, officials said.

But the majority of the Yosemite National Park, including the Yosemite Valley area famous for its towering rock formations, waterfalls, meadows and pine forests, remains open to the public.

Governor Jerry Brown declared a state of emergency for San Francisco on Friday due to threats to the city's water supply.

The blaze has destroyed about a dozen homes and 1,000 outbuildings, and 4,500 additional dwellings remain threatened.

The cause of the wildfire remains under investigation.

The blaze in the western Sierra Nevada Mountains is among the fastest-moving of 50 large wildfires raging across the drought-parched US west.